In general, a liquid developer which is used for development of electrostatic latent images comprises a coloring agent such as carbon black or various kinds of pigments, a coating agent which adheres to or coats over the coloring agent thereby to adjust the charge degree of toner grains, to accelerate the dispersibility thereof and to enhance the fixability thereof after development, a dispersing agent which is dissolved in or swollen by a carrier liquid thereby to enhance the dispersion stability of toner grains, a charge adjusting agent to adjust the amount of the charge of the toner grains and a carrier liquid having a high electric resistance (10.sup.9 to 10.sup.15 .OMEGA..multidot.cm).
Suitable coating agents are, for example, rubbers such as butadiene rubber, styrene-butadiene rubber, cyclic rubber or natural rubber, synthetic resins such as styrene resins, vinyltoluene resins, acrylic resins, methacrylic resins, polyester resins, polycarbonates, polyvinyl acetates or ethylene copolymers, alkyd resins or modified alkyd resins such as rosin resins, hydrogenated rosin resins or linseed oil-modified alkyd resins, and natural resins such as polyterpene resins. In addition, phenol resins and modified phenol resins such as phenol-formaldehyde resins, as well as natural resin-modified maleic acid resins, pentaerythric phthalate, chroman-indene resins, ester gum resins and vegetable oil-polyamides are known. Among them, ethylene copolymers are especially mentioned which are excellent for transfer printing.
JP-A-No. 61-180248 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") mentions an example of using ethylene/methacrylic acid copolymer as an ethylene copolymer. However, as the copolymer has a large internal cohesive force because of the carboxyl group thereof, the dispersibility thereof is still insufficient even with the plasticizing method as suggested therein, where the copolymer is heated at a temperature higher than the softening point thereof in the presence of Isopar L, and therefore the copolymer is hardly formed into fine grains. Because of the poor dispersion stability of the copolymer, the precipitation of the toner grains is noticeable causing various problems in actual development operation that the line images printed are rough and the resolving power is poor. JP-A-No. 62-209543 mentions an example of using a polar solvent as a means of dispersing the copolymer. However, this approach could not be said to be always preferable for a liquid developer containing an electric insulating nonaqueous solvent as a carrier liquid because of the charge characteristic thereof. JP-A-No. 58-129438 mentions a method of preparing toner grains by heating and dissolving an ethylenic copolymer such as an ethylene-vinyl acetate copolymer in a solvent and then cooling the same. However, this method also has some drawbacks that the polymer separates out from the coloring agent during dissolution thereof and the grain size of the grains to be precipitated out during cooling is often nonuniform. As will be noted from the examples, conventional ethylene copolymers have a problem in terms of the dispersibility thereof. Almost no other polymers have hitherto been found out, which have excellent transferability, dispersibility and fixability.
When conventional ethylene copolymers are used as a coating agent for a liquid developer, the dispersibility is poor although the transferring efficiency is good. Accordingly, the dispersion stability is poor and the toner grains precipitate in a short period of time so that the resolving characteristic is insufficient and the image reproducibility is poor. Such are serious problems.